The present invention relates to a rate detecting method and a rate detecting apparatus each for detecting, upon receipt of a signal coded at a predetermined rate, the rate of the coded signal that has been received.
In the field of digital CS broadcasting or the like, a Viterbi decoding method has been used as one of convolutional decoding methods. If a coded signal is received and the rate r (mxe2x89xa6rxe2x89xa6n where m represents a minimum value and n represents a maximum value) of the received signal is unknown to the receiving side, the rate r has been detected conventionally by determining the synchronization of the coded signal by using a Viterbi decoder. In this case, it is sensed whether or not the synchronization can be determined by sequentially varying the frequency of a clock signal for determining the synchronization from the minimum value m of the coded signal to the maximum value n thereof in the receiving apparatus. If the synchronization cannot be determined even when the clock signal reaches the frequency corresponding to the maximum value n of the rate, the clock signal is reset to the frequency corresponding to the minimum value m of the rate and it is judged again whether or not the synchronization can be determined. The foregoing process is repeatedly performed till the synchronization is determined.
In the conventional rate detecting method and apparatus, however, digital noise arises due to a significant change in the frequency of the clock signal which occurs when the clock signal is reset from the frequency corresponding to the maximum value n of the rate to the frequency corresponding to the minimum value m thereof. If the rate detecting apparatus and an analog circuit are integrated in a single substrate as a semiconductor integrated circuit, the problem is encountered that the digital noise adversely affects the performance characteristics of the semiconductor integrated circuit.
It is therefore an object of the present invention to solve the foregoing problem and suppress the digital noise which arises when the synchronizing signal is changed.
To attain the object, the present invention reduces, when the synchronization of the coded signal that has been received is to be determined, the difference between a rate for synchronization determination and another different rate such that it is smaller than a permissible value of the rate determined by the lower and upper limit values of the coded signal that has been received.
Specifically, a rate detecting method according to the present invention assumes a rate detecting method for detecting, in a signal that has been coded at a predetermined rate, the predetermined rate of the coded signal, the method comprising: a first synchronizing signal generating step of generating a first synchronizing signal having a frequency corresponding to a first rate between lower and upper limit values of the rate; a first decoded signal generating step of decoding the coded signal based on the first synchronizing signal to generate a first decoded signal; a first synchronization determining step of determining whether or not desired synchronization is obtainable for the first decoded signal; a second synchronizing signal generating step of generating, if the synchronization of the coded signal is not obtainable in the first synchronization determining step, only a second synchronizing signal having a frequency corresponding to a second rate having a difference between itself and the first rate which is smaller than a permissible value of the rate determined by the lower and upper limit values; a second decoded signal generating step of decoding the coded signal based on the second synchronizing signal to generate a second decoded signal; a second synchronization determining step of determining whether or not the desired synchronization is obtainable for the second decoded signal; and a reiterating step of reiterating, if the synchronization of the coded signal is not obtainable in the second synchronization determining step, the second decoded signal generating step and the second synchronization determining step till the synchronization of the coded signal is obtained, while changing the frequency of the second synchronizing signal such that it is smaller than the permissible value.
In accordance with the rate detecting method of the present invention, only the second synchronizing signal having the frequency corresponding to the second rate having the difference between itself and the first rate which is smaller than the permissible value of the rate is generated in the second synchronizing signal generating step. Accordingly, the difference between the frequency of the first synchronizing signal and the frequency of the second synchronizing signal is reduced to be smaller than the permissible value of the rate determined by the lower and upper limit values, which suppresses digital noise arising when switching is performed from the frequency of the first synchronizing signal to the frequency of the second synchronizing signal.
In the rate detecting method of the present invention, the first decoded signal generating step or the second decoded signal generating step preferably includes the step of decoding the coded signal by using a Viterbi decoding method. This ensures decoding of the coded signal actually (commercially) used.
In the rate detecting method of the present invention, the reiterating step preferably includes the step of changing the frequency of the second synchronizing signal such that it increases stepwise and changing, if the frequency after the change is over the upper limit value of the rate, changing the frequency of the second synchronizing signal such that it decreases stepwise. This allows the reiterating process to be performed positively without changing the frequency in a single step to the lower limit value of the rate if the frequency after the change is over the upper limit value of the rate.
In the rate detecting method of the present invention, the reiterating step preferably includes the step of changing the frequency of the second synchronizing signal such that it decreases stepwise and changing, if the frequency after the change is under the lower limit value of the rate, changing the frequency of the second synchronizing signal such that it increases stepwise. This allows the reiterating process to be performed positively without changing the frequency in a single step to the upper limit value of the rate if the frequency after the change is under the lower limit value of the rate.
A rate detecting apparatus according to the present invention assumes a rate detecting apparatus for detecting, in a signal that has been coded at a predetermined rate, the predetermined rate of the coded signal, the apparatus comprising: a decoding circuit for receiving the coded signal and decoding the coded signal that has been received to output a decoded signal; a synchronizing signal generating circuit for generating a synchronizing signal used to perform synchronization determination with respect to the decoded signal and outputting the synchronizing signal to the decoding circuit; and a synchronization detecting circuit for receiving the decoded signal, performing predetermined synchronization determination with respect to the decoded signal that has been received, and outputting, if the synchronization has not been determined, a frequency switching signal for switching a frequency of the synchronizing signal to the synchronizing signal generating circuit, the synchronizing signal generating circuit generating, based on the frequency switching signal, only a first synchronizing signal having a frequency corresponding to a first rate between lower and upper limit values of the rate in the coded signal and a second synchronizing signal having a frequency corresponding to a second rate having a difference between itself and the first rate which is smaller than a permissible value determined by the lower and upper limit values.
In the rate detecting apparatus of the present invention, only the first synchronizing signal having the frequency corresponding to the first rate between the lower and upper limit values of the rate in the coded signal and the second synchronizing signal having the frequency corresponding to the second rate having the difference between itself and the first rate which is smaller than the permissible value of the rate are generated. Accordingly, the difference between the frequency of the first synchronizing signal and the frequency of the second synchronizing signal is smaller than the permissible value of the rate determined by the lower and upper limit values, which suppresses digital noise arising when switching is performed from the frequency of the first synchronizing signal to the frequency of the second synchronizing signal. Even if the apparatus according to the present invention has another circuit adjacent the peripheral portion thereof, the performance characteristics of the adjacent circuit are not affected thereby.
In the rate detecting apparatus of the present invention, the decoding circuit is preferably a Viterbi decoding circuit for performing decoding in accordance with a Viterbi decoding method. This ensures decoding of an actually used coded signal.
In the rate detecting apparatus of the present invention, the synchronizing signal generating circuit preferably changes the frequency of the second synchronizing signal such that it increases stepwise and changes, if the frequency after the change is over the upper limit value of the rate, the frequency of the second synchronizing signal such that it decreases stepwise. This allows the reiterating process to be performed positively without changing the frequency in a single step to the lower limit value of the rate if the frequency after the change is over the upper limit value of the rate.
In the rate detecting apparatus of the present invention, the synchronizing signal generating circuit preferably changes the frequency of the second synchronizing signal such that it decreases stepwise and changes, if the frequency after the change is under the lower limit value of the rate, the frequency of the second synchronizing signal such that it increases stepwise. This allows the reiterating process to be performed positively without changing the frequency in a single step to the upper limit value of the rate if the frequency after the change is under the lower limit value of the rate.